Paper machine optical monitoring device with integral standardizing optical window

Abstract

In an illustrated embodiment, brightness, color, opacity and fluorescent contribution to brightness are measured by an on-line sensing head providing for simultaneous measurement of transmitted and reflected light. The instrument is designed so as to be capable of transverse scanning of a moving paper web on the paper machine. An optical window member serves as a backing for the web for reflectance measurements and is in series with the web with respect to transmittance measurements. The optical window itself is selected as to its reflectance and transmittance so as to provide for periodic standardization of the instrument in an off-sheet position.

We claim as our invention: 
     
       1. Apparatus for obtaining a quantitative measure of an optical property of a moving web of substantially homogeneous sheet material, which comprises: a. an optical monitoring device having a web receiving region for receiving in operative relation thereto a web of sheet material moving along a web path,   b. said optical monitoring device having an optical system with photometric sensor means capable of providing two essentially independent output signals and with two distinct light energy paths each including light source means, spectral response filter means and said photometric sensor means, said photometric sensor means being responsive to light energy received from the web receiving region after impingement on sheet material in said region,   c. each of said two distinct light energy paths having substantially a common spectral response characteristic sufficient to characterize said optical property but being respectively arranged for collecting said light energy from the web receiving region after impingement on said web under respective substantially differentiated conditions so as to provide respective essentially independent output signals from said photometric sensor means such as to essentially characterize two essentially independent optical response parameters of the sheet material and such as to characterize the optical property with substantially greater accuracy than any characterization of said optical property by either one of such optical response parameters taken by itself,   d. automatic digital computer means connected on line with said optical monitoring device and coupled with said photometric sensor means for receiving therefrom said respective essentially independent output signals in accordance with the respective essentially independent optical response parameters and automatically operable on the basis of said output signals to calculate a quantitative indication of said optical property,   e. said monitoring device including an optical window member disposed on the opposite side of the web receiving region from said light source means, and having an extended web-engaging surface adjacent the web receiving region for slidably supporting the web of sheet material as it moves through said web receiving region, said optical window member comprising a translucent diffusing material of substantial absolute reflectance and of substantial absolute transmittance,   f. one of the light energy paths being a reflectance sensing light path for sensing of reflectance of the sheet material as backed by said translucent diffusing material of said optical window member and the other of said light energy paths being a transmittance sensing light path for sensing the transmittance of the sheet material and the translucent diffusing material of said optical window member in series.   
     
     
       2. Apparatus according to claim 1 with g. means mounting said monitoring device for movement transversely of the web path to sample the reflectance and transmittance of the web of sheet material at different portions of the width thereof, and mounting said monitoring device for movement to an off-web position to one side of the web path such that said translucent diffusing material of said optical window member is clear of the web of sheet material, and   h. means for automatically signalling said digital computer means when the monitoring device is in the off-web position and for causing the computer means to thereupon store the output signals from the photometric sensor means as reflectance and transmittance values for the translucent diffusing material of the optical window member itself exclusive of the sheet material.   
     
     
       3. The method of measuring a contrast ratio opacity of a moving sheet of paper, which comprises: a. directing visible light energy through the sheet from a light source at one side of the sheet,   b. reflecting by means of a light reflecting material on the other side of the sheet a first substantial portion of the transmitted light energy which is transmitted from the light source through said sheet, to direct such first substantial portion of such transmitted light energy back toward and through said sheet, and transmitting through the light reflecting material a second substantial portion of said transmitted light energy,   c. photometrically sensing light energy of said first substantial portion and of said second substantial portion of said transmitted light energy and producing reflectance output which is a function of the reflectance of the light reflecting material and the sheet, and transmittance output signal components of substantial magnitude and together characterizing a contrast ratio Ro/Rg where Ro is the reflectance of said paper as measured over a black backing and Rg is the reflectance of said paper as measured over a white backing.   
     
     
       4. The method of claim 3 where the reflectance Rg is the reflectance of the paper over a white backing having a reflectance value of 0.89. 
     
     
       5. The method of claim 3 which includes filtering the light energy prior to the photometric sensing thereof such that said output signal components represent a response substantially in accordance with the C.I.E. Y spectral response function. 
     
     
       6. The method of claim 3 where the first substantial portion of said transmitted light energy which is reflected back toward said sheet is at least about 35% of said transmitted light energy. 
     
     
       7. Apparatus for measuring a contrast ratio opacity of a moving sheet of paper, which comprises: a. means for directing visible light energy through said sheet from one side thereof,   b. means providing a reflecting region on the other side of said sheet for reflecting a first substantial portion of the transmitted light energy which is transmitted through the sheet, to direct such first substantial portion of such transmitted light energy back toward and through the sheet, and for accomodating transmission through said reflecting region of a second substantial portion of the transmitted light energy which is transmitted through the sheet, and   c. means for sensing light energy of said first substantial portion and for providing a reflectance output signal component of substantial magnitude as a measure of a reflectance parameter of said sheet and light reflecting region and for sensing light energy of said second substantial portion and for providing a transmittance output signal component of substantial magnitude as a measure of a transmittance parameter of said sheet, and   d. means for transmitting a resultant signal magnitude as output which is a function of both said reflectance and transmittance output signal components and is such as to characterize a contrast ratio opacity of said sheet.   
     
     
       8. Apparatus according to claim 7 wherein said reflecting region is provided by a translucent window member having a reflectance such that said first substantial portion corresponds to at least about thirty-five percent of said transmitted light energy. 
     
     
       9. Apparatus according to claim 7 where said reflecting region is provided by a window member of translucent diffusing material. 
     
     
       10. Apparatus according to claim 7 with said reflecting region comprising an optical window of substantial absolute reflectance and transmittance with respect to a Y spectral response function, and having an extended surface, and means for guiding a single thickness of the sheet into conforming stable contact with said extended surface of said optical window. 
     
     
       11. Apparatus according to claim 7 with said reflecting region comprising an optical window of translucent diffusing material having an absolute reflectance value of at least about 35% and an absolute transmittance value of substantial magnitude. 
     
     
       12. Apparatus according to claim 7 with said reflecting region comprising an optical window having an absolute transmittance of substantial magnitude and not more than about 56%. 
     
     
       13. Apparatus according to claim 7 with said reflecting region comprising an optical window having an absolute reflectance value of at least about 70% and a transmittance value of at least substantial magnitude. 
     
     
       14. Apparatus according to claim 7 with said reflecting region comprising an optical window having an absolute reflectance of about 90% and a transmittance value of substantial magnitude. 
     
     
       15. Apparatus for obtaining a quantitative measure of an optical property of a moving web of substantially homogeneous sheet material, which comprises an optical monitoring device having a web receiving region for receiving in operative relation thereto a web of sheet material moving along a web path, said optical monitoring device having an optical system with photometric sensor means capable of providing two essentially independent output signals and with two distinct light energy paths each including light source means, spectral response filter means and said photometric sensor means, said photometric sensor means being responsive to light energy received from the web receiving region after impingement on sheet material in said region, each of said two distinct light energy paths having substantially a common spectral response characteristic sufficient to characterize said optical property but being respectively arranged for collecting said light energy from the web receiving region after impingement on said web under respective substantially differentiated conditions so as to provide respective essentially independent output signals from said photometric sensor means such as to essentially characterize two essentially independent optical response parameters of the sheet material, said monitoring device including an optical window member comprising translucent diffusing material disposed on the opposite side of the web receiving region from said light source means, and exhibiting substantial absolute values of reflectance and transmittance, one of the light energy paths being a reflectance sensing light path for sensing of reflectance of the sheet material where backed by said optical window member and the other of said light energy paths being a transmittance sensing light path for sensing the transmittance of the sheet material and said optical window member in series. 
     
     
       16. Apparatus according to claim 15 with means mounting said monitoring device for movement transversely of the web path to sample the reflectance and transmittance of the web of sheet material at different portions of the width thereof, and mounting said monitoring device for movement to an off-web position to one side of the web path such that said optical window member is clear of the web of sheet material, and means for automatically signalling when the monitoring device is in the off-web position and for causing the output signals from the photometric sensor means to be stored as reflectance and transmittance values for the optical window member itself exclusive of the sheet material. 
     
     
       17. In a method of quantitatively sensing an optical property of a moving sheet, the steps comprising: a. directing visible light energy through a sheet receiving region from one side of such sheet receiving region,   b. reflecting by means of a light diffusing material on the other side of the sheet receiving region light energy which is transmitted through said sheet receiving region to direct light energy back toward said sheet receiving region,   c. photometrically sensing light energy which is directed back toward said sheet receiving region and which is reflected from a sheet at said sheet receiving region and producing a reflectance output signal component of substantial magnitude as a function of the reflectance of the light diffusing material and of the sheet, and   d. producing a transmittance output signal component as a function of light energy which is transmitted through the sheet receiving region but is not reflected by said light diffusing material.   
     
     
       18. The method of claim 17 where light energy which is transmitted through the sheet receiving region in the absence of a sheet and which is not reflected by the light diffusing material is collected to produce a standardizing output signal component for use in conjunction with said reflectance and transmittance output signal components in quantitatively characterizing said optical property. 
     
     
       19. The method of claim 17 which includes filtering the light energy prior to the photometric sensing thereof both in the presence of the sheet and in the absence of the sheet in said sheet receiving region such that the output signal components so obtained represent responses substantially in accordance with the C.I.E. Y spectral response function. 
     
     
       20. The method of claim 17 where light energy which is transmitted through the sheet receiving region in the absence of a sheet and which is reflected by the light diffusing material is collected to produce a standardizing output signal component for use in conjunction with said reflectance and transmittance output signal components in quantitatively characterizing said optical property. 
     
     
       21. The method of claim 20 where the reflectance of the light diffusing material is such that at least about 35% of the light energy incident thereon is reflected therefrom in the absence of a sheet in said sheet receiving region. 
     
     
       22. The method of claim 20 where light energy is transmitted through said light diffusing material in the absence of a sheet in said sheet receiving region and is photometrically sensed to provide a further standardizing output signal component as a contemporaneous measure of the transmittance of the light diffusing material itself for use in conjunction with the first-mentioned standardizing output signal component and said reflectance and transmittance output signal components in quantitatively characterizing such optical property. 
     
     
       23. In a method of quantitatively sensing an optical property of a moving sheet, the steps comprising: a. directing visible light energy through a sheet receiving region from one side of such sheet receiving region,   b. reflecting by means of a light diffusing material on the other side of the sheet receiving region light energy which is transmitted through said sheet receiving region to direct light energy back toward said sheet receiving region,   c. photometrically sensing light energy which is directed back toward said sheet receiving region and which is reflected from a sheet at said sheet receiving region and producing a reflectance output signal component of substantial magnitude as a function of the reflectance of the light diffusing material and of the sheet,   d. producing a transmittance output signal component as a function of light energy which is transmitted through the sheet receiving region but is not reflected by said light diffusing material, and   e. photometrically sensing light energy which is reflected from said light diffusing material in the absence of a sheet in said sheet receiving region to provide a standardizing output signal component as a contemporaneous measure of the reflectance of the light diffusing material itself for use in conjunction with said reflectance and transmittance output signal components in quantitatively characterizing said optical property.   
     
     
       24. Apparatus for measuring an optical property of a moving sheet, which comprises: a. light source means for directing visible light energy through said sheet from one side thereof,   b. a translucent diffusing material disposed for reflecting a portion of the light energy which is transmitted through the sheet, to direct light energy back toward the sheet, while accomodating transmission therethrough of a portion of the light energy which is transmitted through the sheet, and   c. sensing means for sensing light energy as directed back toward the sheet and as reflected from the sheet and for providing a reflectance output signal component of substantial magnitude as a measure of a reflectance parameter of said sheet, and for sensing light energy which is transmitted through the sheet and the light diffusing material in series and for providing a transmittance output signal component of substantial magnitude as a measure of a transmittance parameter of said sheet.   
     
     
       25. Apparatus according to claim 24, with said sensing means being operable for sensing light energy which is reflected from the light diffusing material in the absence of the sheet to provide a standardizing output signal component for use in conjunction with said reflectance and transmittance output signal components in quantitatively characterizing said optical property. 
     
     
       26. Apparatus according to claim 24 wherein said translucent diffusing material itself has an absolute reflectance of at least about thirty-five percent. 
     
     
       27. Apparatus according to claim 24 with said translucent diffusing material being of substantial absolute reflectance and transmittance with respect to a spectral response function for characterizing the optical property, and having an extended surface, and moving sheet guiding means for guiding a single thickness of the sheet into conforming stable contact with said extended surface of said translucent diffusing material. 
     
     
       28. Apparatus according to claim 27 with said translucent diffusing material having an absolute reflectance value of at least about 35% and an absolute transmittance value of substantial magnitude. 
     
     
       29. Apparatus according to claim 24 with said translucent diffusing material having an absolute transmittance of substantial magnitude but not more than about 56%. 
     
     
       30. Apparatus according to claim 24 with said translucent diffusing material having an absolute reflectance value of at least about 70 percent and a transmittance value of at least substantial magnitude. 
     
     
       31. Apparatus according to claim 24 with said translucent diffusing material having an absolute reflectance of about 90% and a transmittance value of substantial magnitude.